Energy storage devices, particularly batteries and supercapacitors, are often optimized for a list of performance criteria, such as energy density, power density, cycling life time, etc. Very rarely are these energy storage devices designed and optimized for other criteria as part of the systems they are intended to power, such as weight and volume. To use conventional energy storage devices in a system, they must be packaged, typically in cylindrical or prismatic form, and attached to structural elements. This not only increases the overall weight and volume of the system, but also may comprise the most effective design.
Lithium ion batteries are widely used in consumer products and have also been evaluated for automotive and aerospace applications. The battery chemistries are optimized to deliver the highest energy density, power density, and cycle life with minimum cost. For simplicity, rechargeable lithium ion batteries are used as an example herein, although the description is analogous in supercapacitors and other battery chemistries.
The cross section of a common lithium ion battery bi-cell (based on Bellcore technology) is shown in FIG. 1. A first cathode 110 is composed of an active material 113 (LiCoO2) and an electrically conducting additive 117 (carbon black) held together in a polymer matrix 118 of polyvinylidene difluoride-hexafluoroproplylene (PVDF-HFP). An aluminum grid 115 is sandwiched within the cathode 110. A porous separator 120 composed of a polymer matrix 128 separates the cathode 110 and the anode 130. For the anode 130, graphite is the active material 133, carbon black is the conducting additive 137, and PVDF-HFP is the matrix 138. Sandwiched within the anode is a copper grid 135. A second separator 140 of PVDF-HFP with liquid electrolyte separates the anode 130 and a second cathode 150, which is constructed similar to the first cathode 110.
In forming the battery 100, after the battery 100 components are laminated together, the entire battery 100 is soaked in a liquid electrolyte, such as LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC). The absorbed liquid electrolyte enables ion conduction through the battery. Copper 135 and aluminum grids 115 and 155 act as current collectors for the anode 130 and the cathodes 110 and 150, respectively.
This battery structure, which is similar to other energy storage devices, dictates several shortcomings that greatly impact its integration into a system. Due to its weak structural properties, the battery 100 is usually encased in a metal protective structure, and once encased, the battery 100 has a fixed geometry which often dictates the system design. In summary, batteries, or other energy storage devices, only function as a power supply to a system and often become a limitation for system design.
There is a growing need to have the components of the energy storage device designed to be the mechanical structure, so all or part of an existing structure in a system can be replaced by the energy storage device. This will allow for greater flexibility in the system design while achieving weight and volume savings. Such devices would be applicable to automotive (vehicle accessory power, hybrid vehicles, etc.), military (micro unmanned air vehicles, unmanned air ship, soldier power, etc.), aerospace products (accessory power for sensors, structural sensors for composites, etc.), and consumer items (portable devices, clothing, fabric, etc.), to name only a few.
There have been several attempts to design power elements that can be better integrated into systems while providing added functionality. Qidwal et al., in “Design and Performance of Composite Multifunction Structure-Battery Materials,” 17th American Society of Composites Conference, Paper #141, October 2002, West Lafayette, Ind., proposed using the aforementioned Bellcore battery technology in more structural geometries, as well as adding additional “inert” layers to improve the battery's structural properties.
The Bellcore battery technology has also been incorporated into the wing of a Micro-UAV, as disclosed by Thomas et al., “Multifunction Structure-Battery Materials for Enhanced Performance in Small Unmanned Air Vehicles,” Proceedings of IMECE2003: International Mechanical Engineering Congress and R&D Exposition, Nov. 15-21, 2003, Washington D.C. This design, however, only provides a better distribution of the battery weight rather than saving weight for the total system.
A power fiber concept originally developed by Armstrong et al., disclosed in published U.S. Patent Application 20030068559, Apr. 10, 2003, herein incorporated by reference, employs vacuum deposition techniques to deposit a coaxial thin film battery structure on a fiber which can then be used to fabricate structural composites. Unfortunately, the thin film battery stores very small amounts of energy and the fabrication process is extremely expensive.
P. C. Lyman used layered batteries or supercapacitors in the shape of a honeycomb structure as the core in a typical sandwich structural composite, in U.S. Pat. No. 5,567,544, issued Oct. 22, 1996, herein incorporated by reference. This concept simply replaces the virtually weightless core of a sandwich structure with batteries to space apart the load bearing face sheets. This concept does not attempt to make the energy storage device itself significantly load bearing and is only applicable where the added volume of a sandwich structure is necessary and/or acceptable. In addition, the use of liquid electrolyte in these power elements greatly limits the application temperature and structure.
Wadley et al., in published U.S. Patent Application 20030049537, published Mar. 13, 2003, have also worked on developing a structural battery by including the necessary active components of a battery within a porous metallic structure which would be the load bearing element. Similar to the work done by Lyman, this concept also relies on a liquid electrolyte.
Thus, what is needed is an inexpensive mechanically robust composite from the battery components without the need for additional structural elements. Also, there is a need to provide a means to monitor the health status of the structure. Current technology requires additional sensors to achieve this level of functionality.